1. A hazard is said to exist when a circuit has the possibility of producing such a glitch. 4.4 Timing Hazards ReturnNext Because of circuit delays, the transient behavior of a logic circuit may differ from what is predicted by a steady-state analysis. In particular, a circuit ’ s output may produce a short pulse, often called a glitch, at a time when steady-state analysis predicts that the output should not change.

2. 4.4 Timing Hazards 1. Static Hazards  A static-1 hazard is a pair of input combinations that: (a) differ in only one input variable and (b) both give a 1 output; such that it is possible for a momentary 0 output to occur during a transition in the differing input variable. x y z 1 1 1010 1010 1010 1010 1010 A static-1 hazard NextBackReturn Simulation

3. 4.4 Timing Hazards  A static-0 hazard is a pair of input combinations that: (a) differ in only one input variable and (b) both give a 0 output; such that it is possible for a momentary 1 output to occur during a transition in the differing input variable. 0 0 1010 1010 1010 1010 1010 A static-0 hazard x y z NextBackReturn

4. 4.4 Timing Hazards 2. Finding Static Hazards Using Maps  A Karnaugh map can be used to detect static hazards in a two-level sum-of-products or product-of-sums circuit.  A properly designed two-level sum-of-products (AND-OR) circuit has no static-0 hazards. It may have static-1 hazards.  A properly designed two-level product-of-sums (OR-AND) circuit has no static-1 hazards. It may have static-1 hazards. NextBackReturn

5. 4.4 Timing Hazards NextBackReturn Examples 111 1 yz x 00 01 11 10 0101 111 1 yz x 00 01 11 10 0101 When z from 1 goes to 0, it is possible for the output to “glitch” momentarily to 0 if y.z from 1 goes to 0 before x.z from 0 goes to 1.

6. 4.4 Timing Hazards  A dynamic hazard is the possibility of an output changing more than once as the result of a single input transition. Multiple output transitions can occur if there are multiple paths with different delays from the changing input to the changing output. 3. Dynamic Hazards z w x y 0 0→1 0 1 slow slower 1→0 1 0→1 1→ 0 0→1→ 0 1→0 1→0→1→ 0 BackReturn